Using vector analysis to assess nitrogen status of ponderosa and Jeffrey pine along deposition gradients in forests of southern California

Abstract

The N status of the dominant pine species ( Pinus ponderosa Laws. and P. jeffreyi Grev. & Balf) was determined at three plots in the San Gabriel Mountains and four plots in the San Bernardino Mountains. Research plots were located along two N deposition gradients. Tree N status was evaluated based on vector analysis of the foliar response to fertilization with ammonium nitrate. Fertilizer was applied at a rate of 500 kg N ha −1 in the spring, and foliage was harvested and analyzed for dry mass, total N concentration and N content (needle mass × N concentration) at the end of the growing season. Unfertilized companion trees located in the vicinity of the fertilized trees served as controls. Vector analysis for Camp Paivika (CP), the westernmost site in the San Bernardino Mountains with the greatest N deposition, supported previous data suggesting that N is not growth limiting, and that available N is in excess of biotic demand. Results were inconclusive from Strawberry Peak (SP), another high-pollution site, albeit with lower N deposition than CP. Foliar N concentration at SP decreased with fertilization, possibly as a result of N dilution owing to foliar biomass increase. At two low-pollution sites on the eastern end of the San Bernardino Mountains, and at three mixed-conifer sites in the San Gabriel Mountains, vector analysis indicated that N was growth limiting. These results tentatively support the hypothesis that chronic N deposition greater than approximately 25 kg ha −1 year −1 in the transverse ranges of southern California has alleviated N deficiency and induced N saturation, whereas N deposition of 6–11 kg ha −1 year −1 has not. At Kratka (KR), a site in the San Gabriel Mountains with bulk throughfall deposition of approximately 11 kg N ha −1 year −1, P. jeffreyi trees were N-deficient based on vector analysis of foliar response to N fertilization. Although relatively high total N levels and NO 3 −:NH 4 + ratios, and low C:N ratios in soil indicated that site N status may be improving at KR, soil N availability is still insufficient to satisfy N demand in Jeffrey pine.